Tube-type heat exchangers are extensively utilized, and these structures normally use a plurality of tubes connected in series and disposed in adjacent parallel relationship. The adjacent tubes are connected by end fittings which are generally of a C-shaped configuration and are commonly referred to as a return or cross-over elbow. The known elbows have been difficult and costly to fabricate, or in the alternative have been unnecessarily large and space consuming. To minimize space, particularly in the lengthwise direction of the heat exchanger, the elbow is normally of a flat channel-like shape and has two substantially parallel legs joined to two 90.degree. bends, which bends in turn are joined by an intermediate straight portion which extends transversely of the legs. Because the two bends are joined by the intermediate straight portion, these elbows have necessarily required that two sequential bending operations be performed thereon, either manually or sequentially on a bending machine. This necessarily increases the complexity of the forming operation, and also introduces dimensional variations in the finished elbow.
While attempts have been made to form the return elbow by a single bending operation, this has necessarily resulted in the finished elbow having a substantially semicircular configuration since the elbow is bent on a constant radius. This is highly undesirable since it results in the length of the elbow being substantially increased, particularly when the adjacent finned heat exchanger tubes are spaced a substantial distance apart. This elbow thus substantially increases the overall length of the heat exchanger and yet does not permit this increased length to be used in a practical manner.
Another problem commonly encountered in the formation of return elbows is the difficulty in putting the desired bends in the tube without causing undesired deformation or damage to the elbow. Many of the known techniques for forming and bending elbows have resulted in the tube wall being at least partially collapsed or radially inwardly deformed, particularly throughout the bent portion. This not only results in irregularities in the tube cross section, but also reduces the flow area through the tube, thereby imposing an undesired flow restriction through the heat exchanger. The known bending techniques have also often caused undesirable wrinkling of the tube wall, particularly on the inner radius of the bend, which weakens the tube and disrupts the optimum flow of heat exchanger fluid.
Accordingly, it is an object of the present invention to provide an improved apparatus for forming and bending return or cross-over elbows, which apparatus overcomes the above-mentioned disadvantages. More specifically, it is an object of the present invention to provide:
1. An apparatus, as aforesaid, which permits the formation of a cross-over elbow having two bend portions joined by an intermediate straight portion, which elbow can be formed by a single bending operation.
2. An apparatus, as aforesaid, which permits the formation of a cross-over elbow in a manner which prevents collapsing, wrinkling, or other undesired deformation of the elbow during the bending process, and which results in an elbow which is of smooth and uniform curvature and possesses a smooth and uniform flow opening therethrough.
3. An apparatus, as aforesaid, which utilizes a mandrel positioned interiorly within the tube stock during bending thereof, which mandrel is progressively withdrawn from the tube stock in correspondence with the progressive bending thereof so that the interior mandrel is always positioned at the point of bending and provides interior support for the tube wall to prevent collapsing thereof during the bending operation.
4. An apparatus, as aforesaid, which permits return elbows to be automatically and repetitively formed at a high rate of production while resulting in elbows of high quality and minimum dimensional variations.
5. An apparatus, as aforesaid, which structurally and functionally is readily adapted to permit the simultaneous bending and formation of a plurality of identical elbows during each bending operation.
Other objects and purposes of the present invention will be apparent to persons familiar with structure of this general type upon reading the following specification and inspecting the accompanying drawings.